backport: bitcoin#18531, #20012, #21035, #21572, #21574 (rpc command) and related fixes

[knst]

Issue being fixed or feature implemented

Backports from bitcoin v22 rpc command related

What was done?

See commits for backports. Also:

• refactored and significantly simplified implementation of composite commands
• added missing changes from bitcoin#19250
• fix help message for rpc help - all subcommands support it now
• add missing client's argument parsing for RPC commands
• CoreContext uses std::monostate instead nullopt which is best-practice for std::variant

How Has This Been Tested?

Run unit/functional tests. Checked autocomplete for various commands Checked help for various commands

Breaking Changes

n/a

Checklist:

• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added or updated relevant unit/integration/functional/e2e tests
• [ ] I have made corresponding changes to the documentation
• [x] I have assigned this pull request to a milestone

[knst]

In theory, this PR can be split to 3...7 PRs, but they can not be reviewed and merged in parallel, they should be done in the specific order one-by-one. So, that's an one PR.

For example, it can be split like that:

1st round: Prior fixes (1PR):

• refactor: use monostate instead std::optional in CoreContext
• fix: missing changes from bitcoin#19250
• feat: fix help message - all subcommands support it now!
• fix: add missing client's argument parsing for RPC commands

2nd round: 3 Backports (1 PR):

• Merge #18531: rpc: remove deprecated CRPCCommand constructor
• Merge #20012: rpc: Remove duplicate name and argNames from CRPCCommand
• chore: add TODO to make client parsing for composite commands

3rd round: Follow-up refactoring (1 PR):

• refactor: simplify implementation of RPC composite commands

4th round: And finally 3 remaining backports (1 PR):

• Merge #21035: Remove pointer cast in CRPCTable::dumpArgMap
• Merge #21572: Fix wrong wallet RPC context set after #21366
• Merge #21574: Drop JSONRPCRequest constructors after #21366

[github-actions[bot]]

This pull request has conflicts, please rebase.

[github-actions[bot]]

This pull request has conflicts, please rebase.